Is there something wrong with MMR?

I'm Gold V and every other game this patch I've had disgustingly mismatched games. The last game I played, my team was Gold V to Gold III in range. Their team? A plat V and a Diamond 5, who both stomped. This bug, if it is one, is making this game a frustration to play. My games are always stompy and unfair because matchmaking put players way above my MMR into my game. If this isn't a bug, what gives? I know it's the preseason, but why does this have to affect MMR and my games?